Zinc-based coated steel sheets, such as hot dip galvanized steel sheets and electrogalvanized steel sheets, have excellent corrosion resistance. However, these types of zinc-based coated steel sheets are inferior to cold-rolled steel sheets in press formability.
Up to the present, many methods of improving the press formability of zinc-based coated steel sheets have been proposed. For example, Unexamined Japanese Patent Publication No. 62-192597 discloses a method of applying an iron-based hard plating on the zinc coating layer. This method prevents galling between the coating layer and the die by increasing the hardness of the material surface. Unexamined Japanese Patent Publication No. 4-176878 discloses a method of improving the frictional property by forming a film containing an oxyate of P or B and a metallic oxide on the surface of the zinc coating layer. These conventional technologies, however, require construction of an additional exclusive-use treatment facility after the normal zinc coating line, which entails the problem of increased cost in manufacturing steel sheets.
Zinc-based coated steel sheets having a zinc phosphate film thereon are known as lubricant coated steel sheets having an excellent frictional property and good press formability. The method used to obtain the zinc phosphate film is called “prephosphate treatment,” which is a method of forming a film by a dipping process, coating process, or the like using an acidic aqueous solution containing ions of zinc, phosphoric acid, nitric acid, fluoride, or the like. This method is applicable to general-purpose treatment facilities. However, in steel sheets having this type of zinc phosphate film, a reaction layer forms between the zinc coating layer and the zinc phosphate film. Since ploughing occurs in the reaction layer when the steel sheet is subjected to sliding, particularly during sliding under imperfect lubrication conditions, the frictional property is reduced and galling tends to occur. Consequently, this method has the problem of a poor frictional property in unlubricated local areas, for example, where the press oil film is broken during press forming. This type of zinc phosphate coating also has poor film removability (detachability) in the alkaline degreasing step, which is a preliminary treatment for painting processes, resulting in formation of a non-uniform phosphate film and degradation of appearance after painting.
Unexamined Japanese Patent Publication No. 9-111473 discloses a zinc-based coated steel sheet having excellent press formability, in which the zinc-based coated steel sheet has a coating composition which contains a compound having a “boundary lubrication function.” The disclosure defines the term “boundary lubrication function” as “a function that the coating composition reacts with the lubricant oil or the surface of the steel sheet triggered by heat and pressure generated at the sliding interface in the press forming step, and bounds therewith, thus preventing the contact of the reaction product with the tool and the surface of the steel sheet.” The disclosure mentions fine particle of phosphate as an example of a compound having the “boundary lubrication function.” The embodiment of the disclosure describe a zinc phosphate film formed by applying and then drying an aqueous solution of zinc phosphate. However, it is the common understanding of persons skilled in the art that zinc phosphate has hard solubility in water, although it readily dissolves in dilute acid. Accordingly, the addition of an acid is essential to obtain an aqueous solution of zinc phosphate, as described in the embodiments. The zinc phosphate film which is obtained by applying and then drying the aqueous solution unavoidably forms a reaction layer between the zinc coating layer and the zinc phosphate film due to etching of the zinc coating layer by the acid component. That is, the technology of the disclosure is within the scope of known prephosphate treatments. Furthermore, since zinc phosphate is an inherently stable compound, the zinc phosphate has only weak performance in forming a reaction products with lubricant oil and with the metal of the steel sheet surface under the heat and pressure conditions which exist during press forming, and thus has substantially no boundary lubrication function.
Therefore, neither of the conventional technologies described above satisfactorily improves the press formability of zinc-based coated steel sheets. Since zinc-based coated steel sheets having a lubricative film are often used as, for example, steel sheets for automotive body panels, the film must also have an excellent anti-peeling property during surface cleaning treatments, such as blank cleaning for press forming.
It would therefore be advantageous to provide at low cost a zinc-based coated steel sheet having a dry film lubricant which has an excellent anti-peeling property in the blank cleaning step, as a preliminary treatment for press forming, and also has an excellent frictional property during press forming, particularly when some part of the material is in a non-lubricated condition, and an excellent anti-galling property, and to provide a method for the manufacture thereof.
It would also be advantageous to provide a zinc-based coated steel sheet having a lubricative film that has, in addition to the characteristics described in the first object, excellent film removability in the alkaline degreasing step, as a preliminary treatment for painting, and has excellent surface appearance after painting, and to provide a method for the manufacture thereof.
It would further be advantageous to provide a zinc-based coated steel sheet that does not deteriorate the film removability of the film in the alkaline degreasing step, and that has an excellent anti-peeling property in the blank cleaning step, and to provide a method for the manufacture thereof.